Recent advances in medicine have extended the life span of human beings and as a result, age-related acute and chronic neurological diseases, such as Alzheimer's disease, stroke, Parkinson's disease etc. increase. These neurological diseases are characterized by the progress of degeneration of specific neurons over the course of diseases. As mature neurons do not regenerate once they die, neuronal death in neurological diseases above can result in incurable loss of essential brain function including cognition, sensation, and movement and thus economic and social overload.
Exicitotoxicity, oxidative stress, and apoptosis have been implicated as major routes of neuronal death occurring in various neurological diseases and propagate through distinctive signaling pathways for each route.
Glutamate is the excitatory neurotransmitter mediating slow excitatory synaptic transmission through N-methyl-D-aspartate (NMDA) receptors and fast excitatory synaptic transmission through kainate or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptors. In the resting state of neurons, Mg2+ blocks NMDA receptor channels in a voltage-dependent manner. With stimuli causing membrane depolarization, Mg2+ is liberated from the NMDA channels, rendering the channels permeable to Ca2+ and Na+. Activation of NMDA receptors plays an important role in physiological process including learning and memory [Siegel G. J. et al., Basic Neurochemistry, 6th edition, Lippincott Williams & Wilkins, 315–333 (1999)]. Besides physiological roles, brief and excess activation of NMDA receptors can cause rapidly evolving neuronal death and mechanisms underlying NMDA receptor-mediated neurotoxicity have been extensively studied over the last tow decades.
In 1969, Olney et al. reported that oral administration of monodium glutamate produced neuronal cell death in brain of mice or monkey [Olney, J. W. and Sharpe, L. G., Science, 166:386–388(1969); Olney, J. W. and Ho, O. L., Nature, 227(258): 609–611(1970)], suggesting that glutamate, the excitatory neurotransmitter, mediates neuronal excitability and death in epilepsy [Olney, J. W., Int. Rev. Neurobiol., 27:337–62:337–362(1985)]. Administration of glutamate induces neuronal death in cultured cortical neurons, which occurs through activation of NDMA receptors and depends upon Ca2+ entry [Choi, D. W., J. Neurosci., 7(2)369–379(1987)]. Glutamate neurotoxicity (or excitotoxicity) has been proposed as a main pathway to neuronal death in stroke as well as epilepsy [Choi. D. W., Neuron, 1:623–634(1988)]. Interrupted blood supply to brain results in deprivation of oxygen and glucose, which causes energy (ATP) failure, dysfunction of ATP-dependent ion channels, and membrane depolarization that increases glutamate release. Energy failure also reduces glutamate uptake into glial cells. Consequently, glutamate is abnormally accumulated in the synaptic cleft [Choi, D. W. and Rothman, S. M., Annu. Rev. Neurosci., 13:171–182(1990); Benveniste, H et al., J. Neurochem., 43(5):1369–1374(1984)]. The excess accumulation of glutamate causes neuronal cell death primarily through activation of NMDA receptors. In fact, administration of NMDA receptor antagonists have been reported to reduce neuronal death following hypoxic-ischemic brain injury [Goldberg, M. P. et al., J. Pharmac. Exp. Ther., 243:784–791(1987); Simon et al., Science 226:850–852 (1984); Sheardown, M. J. et al., Science 247:571–574(1990)].
Extensive evidence supports that excitotoxicity also contributes to neuronal death in neurodegenerative diseases. The key pathological features of Huntington's disease (HD) include degeneration of GABAergic neurons and selective sparing of NADPH diaphorase-containing neurons in the striatal area. These pathological features of HD are observed following the intrastriatal injections of NMDA or quinolinic acid, an NMDA receptor agonist [Ferrante, R. J et al., Science, 230(4625):561–563(1985); Beal, M. F. et al., Nature, 321(6066) :168–171(1986); Koh, J. Y. et al., Science, 234(4772):73–76(1986)]. Amytrophic lateral sclerosis (ALS) is accompanied by degeneration of upper and lower motor neurons and marked by neurogenic atrophy, weakness, and fasciculation. While the pathogenesis of ALS remains to be resolved, excitotoxicity has been expected to participate in the process of the ALS. In particular, ALS patients show defects in synthesis and transport of glutamate and increased levels of extracellular glutamate [Rothstein, J. D., Clin. Neurosci., 3(6):348–359(1995); Shaw, P. J. and Ince, P. G., J. Neurol., 244 Suppl 2:S3–14(1997)].
Although NMDA receptor-mediated excitotoxicity plays a causative role in stroke and neurodegenerative diseases, the therapeutic potential of NDMA receptor antagonists has been limited by unexpected side effects in brain. In particular, systemic administration of NMDA receptor antagonists impairs normal brain function and can cause widespread neuronal damage in adult rat brain [Olney et al., Science 244:1360–1362 (1989)]. The neuropsychopathological side effects are produced by high-affinity NMDA receptor antagonists such as phencyclidine and related NMDA receptor antagosints such as MK-801 (dizocilpine maltate), tiletamine and ketamine and may be overcome with administration of channel-blocking NMDA receptor antagonists with low affinity and rapid-kinetic response [Rogawski, Amino Acids 19:133–149 (2000)].
Free radicals mediate neuronal death occurring in neurological diseases as well as tissue damage occurring in the whole body [Halliwell, B. and Gutteridge, J. M., Mol. Aspects. Med., 8(2):89–193(1985); Siesjo, B. K. et al., Cerebrovasc. Brain Metab. Rev., 1(3):165–211(1989); Schapira, A. H., Curr. Opin. Neurol., 9(4)260–264(1996)]. Free radicals are produced in degenerating brain areas following hypoxic-ischemia or traumatic brain and spinal cord injuries. Antioxidants or maneuvers scavenging free radicals attenuate brain damages by hypoxic-ischemia or traumatic injuries [Flamm, E. s. et al., Stroke, 9(5):445–447(1978); Kogure, K. et al., Prog. Brain Res., 63:237–259(1985); Chan, P. H. J. Neurotrauma., 9 Suppl 2:S417–423(1992); Faden, Pharmacol. Toxicol. 78:12–17 (1996)]. Extensive evidence supports that free radicals are produced in brain areas undergoing degeneration in neurodegenerative diseases possibly due to point mutations in Cu/Zn superoxide dismutase in ALS [Rosen et al., Nature 362:59–62 (1993)], the decrease of reduced glutathione, glutathione peroxidase, and catalase, and the increase of iron in substatia nigra in Parkinson's disease [Sofic, E. et al., J. Neural Transm., 74:199–205(1988); Fahn, S. and Cohen, G., Ann. Neurol., 32(6):804–812(1992)], the oxidation of lipid, nucleotides, and protein, an increase of iron in degenerating neural tissues, and generation of free radicals by beta amyloid in Alzheimer's disease brain [Schubert, D. et al., Proc. Natl. Acad. Sci. U.S.A. 92(6):1989–1993(1995); Richardson, J. S. et al., Ann. N. Y. Acad. Sci. 777:362–367(1996)], and mitochondrial dysfunction in HD [Dexter, D. T. et al., Ann Neurol. 32 Suppl:S94–100(1992)]. Accordingly, antioxidants have been neuroprotective against such neurodegenerative diseases [Jenner, Pathol. Biol.(Paris.) 44:57–64 (1996); Beal, Ann. Neurol. 38:357–366 (1995)].
Zinc (Zn2+) is a transition metal which is highly present and plays a dynamic role in brain. Within cells, zinc is associated with metalloproteins to control the enzymatic activity and structural stability of the proteins. Also, zinc regulates gene expression by binding to various transcription factors. In the CNS, zinc is localized at the synaptic terminal of glutamatergic neurons, released in an activity-dependent manner, and regulates activity of various neurotransmitter receptors and ion channels.
Zn2+ mediates neurodegenerative process observed in seizure, ischemia, trauma, and Alzheimer's disease (AD). The central administration of kainate, a seizure-inducing excitotoxin, causes the translocation of Zn2+ into postsynaptic degenerating neurons in several forebrain areas. Translocation of zinc into adjacent neurons was also observed following ischemic and traumatic brain disease, and the blockade of its transition inhibited neuronal cell death [Frederickson, C. J. and Bush, A. I., Biometals. 14:353–366(2001); Weiss et al., Trend. Pharmacol. Sci. 21:395–401(2001)]. Zinc has been known to enter neurons through Ca2+ permeable NMDA and AMPA/KA receptors, voltage-gated Ca2+ channel, or zinc transporter protein, and to induce neuronal death by the activation of NADPH oxidase generating reactive oxygen species. Zn2+ is observed in the extracellular plaque and degenerating neurons in AD, which likely contributes to neuronal degeneration in AD [Suh et al., Brain Res. 852:274–278(2000); Bush et al., Science 265:1464–1467 (1994); Lee et al., Proc. Natl. acad. Sci. U.S.A. 99:7705–7710(2002)]. Therefore, the inhibition of release and toxicity of zinc has been suggested as new strategy of prevention and treatment for Alzheimer's disease [Fredrickson and Bush, Biometals;, 14:353–66(2001)].
As described above, NMDA receptor-mediated excitotoxicity, oxidative stress, and zinc can contribute to neuronal death in various acute and neurodegenerative diseases in the nervous system. Thus, efficient therapeutic drugs preventing each route of neuronal deaths should be developed to treat such catastrophic neurological diseases.
We have investigated to develop neuroprotective drugs with multiple neuroprotective effects against excitotoxicity or oxidative stress and succeeded in inventing tetrafluorobenzyl derivatives that can be applied to treat stroke, trauma, and some neurodegenerative diseases.